Cellulose based film structure and method for producing the same

ABSTRACT

According to an example aspect of the present invention, there is provided a cost-effective method of producing cellulose based films by introducing an intense water removal system to the process, and cellulose based films thereof having improved properties.

FIELD

The present invention relates to a method for producing good qualitycellulose based films with low production and investment costs byapplying novel intense water removal system, and to an improved filmproduced by such method.

BACKGROUND

In order to produce good quality CNF/CMF films, the film should becasted by applying a low casting solid content. Therefore there is ahigh amount of water to be removed from the fiber web. Typically, theexcess water is removed by evaporation and due to a low solid contentthis leads to high investment and operation costs in the productionline. If water removal by vacuum or pressing is applied, the filmstructure is attached to a fabric and due to low film strengthproperties the film structure is damaged during web detachment from thefabric.

In the prior art closest technology relates to PVA or LCD film castinglines, wherein mirror polished metal belts are applied and heated byapplying hot air. For example U.S. Pat. No. 4,124,677 describes a methodfor manufacturing a polyvinyl alcohol films and Siemann (2005) describestypical methods for solvent casting. The problem with these traditionaltechnologies is that the heating of metal belt with hot air isinefficient and thus leads to high production and investments costs. Itis for example expensive to produce hot air, and furthermore air isincapable of transmitting enough heat to the metal belt. Air heattransfer to metal belt is also uneven, which leads to difficulties inthe process and the products obtained.

Thus, there is need for novel and efficient processes, which consumeless energy and produce commercially usable good quality films from forexample nanocellulose.

SUMMARY OF THE INVENTION

The invention is defined by the features of the independent claims. Somespecific embodiments are defined in the dependent claims.

According to a first aspect of the present invention, there is provideda method for producing continuous cellulose nanofibril (CNF) orcellulose microfibril (CMF) films by introducing a novel intense waterremoval system.

According to a second aspect of the present invention, the presentinvention provides means for improving the quality of the produced CNFor CMF films.

These and other aspects, together with the advantages thereof over knownsolutions are achieved by the present invention, as hereinafterdescribed and claimed.

The method according to an embodiment of the present invention is mainlycharacterized by what is stated in the characterizing part of claim 1.

The film material according to an embodiment of the present invention ismainly characterized by what is stated in claim 11.

Considerable advantages are obtained by means of the invention. It isprovided herein a novel solution, wherein the film is preferablystrengthened before, after of simultaneously with casting, after whichan intensive water removal is applied. These steps lead to significantdecrease in film production investment and operation costs and also toimproved film strength and quality properties. For example theproduction lines can be made essentially shorter than in the traditionalprocesses, and the energy amount typically used for drying the productscan be significantly decreased, since a significant amount of moistureis removed before the actual drying step. Also the resultingend-products are of better quality and easily detachable from thesupport material.

Next, the present technology will be described more closely withreference to certain embodiments.

EMBODIMENTS

The present technology describes a novel method of producing CNF or CMFfilms, wherein the film is preferably strengthened and intensive waterremoval is applied.

FIG. 1 is a simplified process scheme comprising 1) casting, 2)adjustable steam heating of metal belt, 3) adjustable impingementdrying, and 4) web reeler.

FIG. 2 is a simplified process scheme describing another possiblefiguration of the production line, wherein section 1) a strengtheninglayer is being produced, section 2) is responsible for the CNF or CMFlayer casting, in section 3) both layers are being combined and waterremoval is applied, and section 4) comprises web drying and smoothing ofthe produced film.

FIG. 3 is a simplified process scheme describing a metal belt pressingand drying machine with a novel heating and pressing chamber andefficient drying for CNF or CMF film manufacturing.

FIG. 4 shows two comparative images of films produced on a metal belt,wherein the surface of the belt has not been treated (a) and (b) hasbeen treated. In (a) the edges of the film tear when the film is driedand the adhesion (3-5 N/m) causes damages to the film when detached fromthe support. In (b) the edges are clean and the sample is detached fromthe support without damages (adhesion 1-3 N/m).

According to a preferred embodiment of the present invention, the methodfor producing continuous cellulose nanofibril (CNF) or cellulosemicrofibril (CMF) film comprises the steps of:

-   -   casting of CNF or CMF material layer onto a metal belt, fabric        or felt having total length of 10 to 150 m,    -   removing at least 30% of the moisture of the CNF or CMF material        by an intense water removal system, and    -   final drying and reeling of the formed film.

According to one embodiment of the present invention, the metal belt,fabric or felt has total length of 10 to 50 m, or even shorter. Shortcasting lines provide additional savings to the investment costs, andare still proven to result a desired product.

According to a further embodiment of the present invention, the intensewater removal system used in the method comprises one or more of thefollowing steps (a) to (c):

-   -   (a) slurry temperature increment prior to CNF or CMF material        contact with the metal belt, fabric or felt to reach CNF or CMF        slurry dryness of 5 to 30% and temperature of 30 to 90° C.,    -   (b) heated low pressure pulse for gentle water removal right        after the CNF or CMF material contact with the metal belt,        fabric or felt to reach CNF or CMF dryness of 20 to 50%, and    -   (c) applying heat to the metal belt from underneath the metal        belt right after the film contact with the metal belt in        combination with applying impingement drying from above the        metal belt after casting,        or any combination of (a), (b) and (c).

It is preferred that the heating medium is steam instead of e.g. air,for example in a low pressure steam chamber. Steam heat is transferredby condensation, which enables more even distribution of the heatcompared to air heat.

The impingement drying comprises at least a first blowing speed and asecond blowing speed, wherein the first blowing speed is smaller thanthe second blowing speed. Suitable first blowing speed may be forexample 10-30 m/s, whereas the second blowing speed may be for example30-100 m/s.

According to one embodiment of the present invention the intense waterremoval system enables a removal of more than 30%, preferably 30 to 50%of the moisture content of the CNF or CMF material prior to finaldrying.

According to another embodiment of the present invention a strengtheninglayer is produced on top of the CNF or CMF material before, after orsimultaneously with the casting. The strengthening layer consists oflonger fibers than the CNF or CMF material. Fibers may be cellulosefibers, such as cellulose nanofibers, but also any other fibers may beused, which preferably have length of 0.1-1 mm.

According to further embodiment of the present invention, thestrengthening layer requires at least one filler material, whichprovides a compact structure together with the fibers. It is alsocharacterizing to the present invention that the strengthening layer isthinner than the CNF or CMF layer. However, it is preferred that thelong fiber content of the strengthening layer is bigger compared to theCNF or CMF layer in order to achieve desired wet/dry strength.

Wet/dry strength and stretchability of the web can be highly improvedwhen longer fibers are introduced. Longer fibers can stretch andstraighten, which improves handling and further processing properties ofthe web. On the other hand, longer fibers are not sticking andpenetrating into felt or fabric structure during water removal andtherefore the web can be detached with a lower force from the fabricsurface, which leads to better web quality.

The filler material needs to be finely distributed so that it fills thepores of the fibers, causing a minimum integration of the CNF or CMFlayer into the strengthening layer and resulting that the CNF or CMFlayer can be kept thin as well. This enables low material costs, becausetypically CNF or CMF is the most expensive part of the film structure.One example of a suitable filler material is rock.

According to one embodiment of the present invention the strengtheninglayer is adjusted to thickness of 5 to 30 μm and the CNF layer hasadjusted to thickness of 10 to 100 μm. However, strengthening layer isnot necessarily required, but has shown to improve the quality of thefinal product.

Metal belt surfaces can be applied to produce different types of websand other products. Different products need different adhesion to themetal belt surface. To adjust the adhesion, a controllable surfaceand/or coating is needed preferably having good wearing properties. Bycontrolling the surface adhesion it is possible to improve the webproperties and machine runnability.

According to one embodiment of the invention the surface of the metalbelt has been roughened and possibly further coated with flexiblematerial having controlled hydrophobicity and wearing resistant filler.The surface of the metal belt may for example be roughened by grindingand/or blasting to value of Ra 1-3 μm, Rz 1-20 μm, before applying thecoating layer onto it. One suitable coating layer comprisesfluoropolymer particles. After the coating material has been applied, itneeds to be polished. One suitable polishing value is Ra 0.01-1 μm, Rz0.1-8 μm, depending on the aimed end-use and required adhesionproperties thereto.

According to one preferred embodiment of the present invention theadhesion between the film and the metal belt is adjusted to 0.5-5 N/m.It is important that the adhesion is adjusted so that the web remainsattached to the metal belt during the production process even if theline is running with high speed, but the web also needs to be gentlydetachable from the belt surface without damaging the web when the webis being collected and transferred to further processing. The inventorsof the present invention have found out that the adhesion level ashereinbefore disclosed and achieved result the desired effects.

According to one embodiment of the present invention, fabric or felt isused instead of metal belt.

In one embodiment of the present invention, after casting of CNF or CMFmaterial layer onto a fabric or a felt, the film is transferred onto ametal belt for intense water removal according to for example FIG. 2.

In a further embodiment of the present invention, a fabric or a feltcomprises fibers having diameter of 1 to 2000 μm in at least two layers,which are on top of each other. In addition, the felt may comprise atleast one type of slivers having smaller diameter than the fibers. Thefabric preferably has a thickness of 1 to 4 mm and the felt preferablyhas a thickness of 2 to 6 mm. Air permeability of the fabric ispreferably at least 300 cfm, and of the felt preferably at least 1m³/m²/min at 100 Pa in a non-pressurized space.

In one embodiment of the present invention, the layers of a fabric or afelt facing the CNF or CMF layer are denser and/or have smaller openvolume than the other side of the fabric or the felt.

According to a further embodiment the present invention can be appliedfor processes running with high speeds. Such speeds may be for exampleover 500 m/min, or even higher, for a film structure having a basisweight of 20-200 g/m² and width of 3-10 m.

It is to be understood that the embodiments of the invention disclosedare not limited to the particular structures, process steps, ormaterials disclosed herein, but are extended to equivalents thereof aswould be recognized by those ordinarily skilled in the relevant arts. Itshould also be understood that terminology employed herein is used forthe purpose of describing particular embodiments only and is notintended to be limiting.

Reference throughout this specification to one embodiment or anembodiment means that a particular feature, structure, or characteristicdescribed in connection with the embodiment is included in at least oneembodiment of the present invention. Thus, appearances of the phrases“in one embodiment” or “in an embodiment” in various places throughoutthis specification are not necessarily all referring to the sameembodiment. Where reference is made to a numerical value using a termsuch as, for example, about or substantially, the exact numerical valueis also disclosed.

As used herein, a plurality of items, structural elements, compositionalelements, and/or materials may be presented in a common list forconvenience. However, these lists should be construed as though eachmember of the list is individually identified as a separate and uniquemember. Thus, no individual member of such list should be construed as ade facto equivalent of any other member of the same list solely based ontheir presentation in a common group without indications to thecontrary. In addition, various embodiments and example of the presentinvention may be referred to herein along with alternatives for thevarious components thereof. It is understood that such embodiments,examples, and alternatives are not to be construed as de factoequivalents of one another, but are to be considered as separate andautonomous representations of the present invention.

Furthermore, the described features, structures, or characteristics maybe combined in any suitable manner in one or more embodiments. In thefollowing description, numerous specific details are provided, such asexamples of lengths, widths, shapes, etc., to provide a thoroughunderstanding of embodiments of the invention. One skilled in therelevant art will recognize, however, that the invention can bepracticed without one or more of the specific details, or with othermethods, components, materials, etc. In other instances, well-knownstructures, materials, or operations are not shown or described indetail to avoid obscuring aspects of the invention.

While the forgoing examples are illustrative of the principles of thepresent invention in one or more particular applications, it will beapparent to those of ordinary skill in the art that numerousmodifications in form, usage and details of implementation can be madewithout the exercise of inventive faculty, and without departing fromthe principles and concepts of the invention. Accordingly, it is notintended that the invention be limited, except as by the claims setforth below.

The verbs “to comprise” and “to include” are used in this document asopen limitations that neither exclude nor require the existence of alsoun-recited features. The features recited in depending claims aremutually freely combinable unless otherwise explicitly stated.Furthermore, it is to be understood that the use of “a” or “an”, thatis, a singular form, throughout this document does not exclude aplurality.

The word film in this document refers also to other possible web typeslike fiber webs (paper, tissue, board), membranes, printed electronicplatforms, composites etc.

INDUSTRIAL APPLICABILITY

At least some embodiments of the present invention find industrialapplication in areas relating to film manufacturing and packagingindustry, such as for barrier applications, membranes, medical or foodpackaging use, automotive applications, and printed electronics.

EXAMPLES Example 1—Preparation of Cellulose Nanofiber Material PriorCasting

CNF/CMF material slurry having solids of 4-20% was mixed withstretchability and adhesion properties improving additives (e.g.sorbitol, sizes and starch). Part of the water and/or air bubbles wereremoved from the slurry by pressure filtering and vacuum mixing to reacha solids level of 5-30% prior to casting. In combination with this step,temperature of the slurry can be increased applying steam heating to40-90° C. to improve material flow, water removal, evenness, and fastevaporation properties.

Example 2—Preparation of Strengthening Layer

CNF/CMF film layer was precise casted on metal belt surface.Strengthening material layer was manufactured by precise casting onCNF/CMF layer and/or by pressure feeding the strengthening material ontoa permeable fabric, through which the excess water is removed by vacuumand/or pressing elements to form the strengthening layer. Strengtheningmaterial consists mainly of 0.1-1 mm long fibers and possible fillermaterial and possible strengthening additives (e.g. sizes, starch andsorbitol). Strengthening layer and CMF/CNF film layers were combinedprior or after the intensive water removal stage in order to produce astrengthened film. Sizes or other additives can be applied in togetherwith the connection of the layers to improve properties of the formedstrengthened film.

Example 3—Surface Treatment of Metal Belt for Controlled Web Adhesion

Metal belt surface was treated with grinding stone or belt to formmainly machine direction or 5-50° angle compared to machine directionoriented grooves that have depth and width of 0.01-20 μm. Adhesionbetween the metal belt and the web was adjusted by changing the amount,size, and orientation of the grooves. In addition to the previous,ground surface was coated with 1-50 μm coating thickness flexiblematerial having controlled hydrophobicity and wearing resistant filler.The coated surface can also be ground to surface roughness of Ra 0.01-1μm, where the final surface will consist of metal (ground metal beltsurface top areas) and hydrophobic coating (ground metal belt surfacegroove bottom areas). The adhesion between the film and the metal beltwas adjusted to a level of 0.5-5 N/m.

Example 4—Method for Producing Cellulose Nanofiber Film

CNF/CMF film was produced by a method including one or more of thefollowing steps:

-   -   Water removal and/or CNF/CMF temperature increment prior to        casting to reach casting solids of 5-30% applying a pressurized        filtering and a continuous shear force mixing method and/or        heating the slurry using steam,    -   In MD and CD direction precise controlled casting unit head was        applied to form an even film layer on metal belt,    -   Metal belt was heated by adjustable low pressure steam where        steam was condensed on the back surface of the metal belt and        the starting point of the heating was arranged close to the        point where the film is casted on the metal belt,    -   Water was removed from the film by a low pressurized and steam        heated water removal zone that has an effective length of at        least 1 m,    -   Water was removed from the film by an adjustable impingement        drying where below the film solids of 40%, the air speed is        10-30 m/s and above solids of 40% the air speed is 30-100 m/s,    -   Film was smoothened and thickness profile was adjusted applying        a smooth roll in contact with the film where the smoothing roll        is arranged against the metal belt or in a step where a pair of        smoothing rolls are arranged after the metal belt drying,    -   Film was detached or reeled after the metal belt drying step.

CITATION LIST Patent Literature

-   U.S. Pat. No. 4,124,677

Non Patent Literature

-   Ulrich Siemann, Solvent cast technology—a versatile tool for thin    film production, Progr Colloid Polym Sci (2005), vol. 130, pp. 1-14,    DOI: 10.1007/b107336.

1. A continuous cellulose nanofibril (CNF) or cellulose microfibril(CMF) film having a thickness of 10-100 μm, and comprising astrengthening layer having fibers of length of 0.1 to 1 mm, wherein athickness of the strengthening layer is 5 to 20 μm.
 2. The CNF or CMFfilm according to claim 1, wherein the strengthening layer comprisescellulose fibers.
 3. A film comprising: a cellulose nanofibril (CNF) orcellulose microfibril (CMF) layer comprising a CNF or CMF material, anda strengthening layer on a surface of the CNF or CMF layer, thestrengthening layer comprising at least one filler material andcomprising longer fibers than the CNF or CMF material, wherein thestrengthening layer is thinner than the CNF or CMF layer.
 4. The filmaccording to claim 3, wherein the CNF or CMF layer comprises a thicknessof 10-100 μm.
 5. The film according to claim 4, wherein thestrengthening layer comprises a thickness of 5 to 30 μm.
 6. The filmaccording to claim 3, wherein the fibers of the strengthening layercomprise cellulose fibers.
 7. The film according to claim 3, wherein thefibers of the strengthening layer have a length of from 0.1 to 1 mm. 8.The film according to claim 7, wherein the fibers of the strengtheninglayer having a length of from 0.1 to 1 mm comprise cellulose nanofibers.9. The film according to claim 3, wherein the film comprises a cellulosenanofibril (CNF) layer.
 10. The film according to claim 3, wherein thefilm comprises a cellulose microfibril (CMF) layer.